How To Make and Use a Hartmann Mask to Aid FocusThe Hartmann mask and Scheiner Disk are simple devices for making focusing a lens easier, especially a telescope.
A Hartmann mask, named after German astronomer Johannes Franz Hartmann, is a tool used in astronomy to test and calibrate optical devices, notably telescopes. It is a simple yet useful instrument that produces a characteristic diffraction pattern to aid in the accurate focussing of optical components such as mirrors or lenses.
The Hartmann mask is made of a solid plate with uniformly spaced holes or slots set in a specified pattern. Typically, these holes are placed equidistant from the others. The mask is placed over the telescope's aperture, and the diffraction pattern is observed.
The Hartmann mask works on the idea that the diffraction pattern formed by the holes or slits can be used for evaluating the focus of telescopes optical components. When the lenses or mirrors are properly focussed, the diffraction pattern appears symmetrical and centred. Misalignment, focus errors or optical aberrations cause the diffraction pattern to change or become deformed.
A Scheiner Disk is a mask with two holes, whereas a Hartman mask has many holes (typically three).
Hartman mask requires that you target a point of light, such as a star or distant planet, and therefore are not good for day-to-day use, although they can work on the moon.
The Hartmann mask can use either a circular hole or a triangular hole. The triangular hole creates diffraction spikes which are easier to detect when in focus, but the circular holes will allow more light to enter the lens resulting in a clearer image. Once your telescope is in focus, the mask can be removed.
I haven't been in astronomy very long, but a Hartmann mask is one of the most useful accessories that I have used. It makes focusing so much easier and sharp images can be had in seconds. What makes it even better is that you can make one for free!
How to Make a Hartmann Mask
My mask is constructed from an old Amazon box. I tacked down the printed template with sticky tape and cut it out around the perimeter, leaving an approx 2cm gap around the edge. This is to allow for the rim to be created.
Next I cut out the three triangles with a Stanley knife, and then cut slits into the 2cm border and fold them up. I taped these up with Duck Tape to allow the mask to slot over the telescope aperture.
I spray painted the bask matte black to avoid any reflections caused by the shiny Duck tape which may create interference which may affect focusing.
Finally I used some 3M double-sided tape to stick foam pads to the inside edge to keep the mask in place on the telescope.
Refinements to the Hartmann Mask Design
You could have a mixture of circles for light gathering and triangles for diffraction. I imagine this would create an interesting pattern and I will try it sometime.
How to Use a Hartmann Mask
To quickly get the best possible focus, follow these steps to use your new Hartmann Mask.
- Set your camera ISO or gain high and turn on live view.
- Aim your telescope at a star. If you cannot see stars in the live view screen and everything is connected properly, it could be that the ISO or gain needs increasing further, the telescope is so out of focus that they are blurred - try spinning the focuser to see if anything becomes visible. It could also be that the star you are pointing to isn't bright enough. Not much light gets through the mask so be sure to focus using a very bright star such as Vega, Sirius or Arcturus.
- You will see three blurry circles. Adjust focus until these merge into one single point of light.
- Use shutter priority or bulb mode to capture the point of light. Live view is good, but for the most accurate focusing it's always best to try some test images.
- Repeat the previous two steps until you have perfect focus. Perfect focus is when zooming into the point of light on the test photo, it is one single point, not three close together.
